PROJECT SUMMARY (See instructions); Drug abuse by pregnant women is a problem. About 5% of pregnant women take illicit drugs and up to 20% smoke. Drug abuse is often associated with depression, which is also common in pregnant women. Estimated 5-13% of pregnant women take antidepressant drugs. Two cytochrome P450 enzymes, CYP2D6 and CYP2B6 are responsible for the clearance of majority of common antidepressants and drugs of abuse. Most importantly, CYP2B6 is a major enzyme clearing bupropion, an antidepressant drug also used for smoking cessation and, together with CYP2D6, is responsible for the clearance of many stimulants and opiates such as MDMA and methadone. Despite this clinical importance, the changes in CYP2B6 activity and in bupropion clearance during human pregnancy are not known, whereas increased activity of CYP2D6 during human pregnancy is well documented. In contrast, induction of CYP2B6 in vitro by estrogens has been demonstrated whereas mechansims responsible for CYP2D6 induction are not known. The long term objective of this proposal is to establish the mechanisms that contribute to CYP2D6 induction during pregnancy and determine whether the changes in P450 activity during pregnancy can be predicted from in vitro using CYP2B6 and bupropion as a model system. In our specific aim 1 we will test the hypothesis that increasing estrogen concentrations during human pregnancy induce CYP2B6 activity and expression via 1) using human hepatocytes to predict CYP2B6 induction and bupropion and methadone disposition during pregnancy by estrogens and via ER activation and 2) determining bupropion pharmacokinetics during human gestation in comparison to postpartum. In the second aim we will test the hypothesis that CYP2D6 activity increases during pregnancy due to increased CYP2D6 mRNA and HNF4a and RAR activation via 1) determining the disposition of dextromethorphan during pregnancy in CYP2D6 humanized mice and measuring CYP2D6 mRNA and activity during gestation 2) characterizing the impact of HNF4a and RAR activation in CYP2D6 mRNA and activity in human hepatocytes. We will also establish the in vitro parameters needed for modeling and simulation of MDMA and its metabolite disposition during pregnancy.